lotus



previous page: 2d. My HCG level is not rising as quickly as it is supposed to? What is the normal range, and how worried should I be?
  
page up: Pregnancy Screening FAQ
  
next page: 2.a What is apha-fetoprotein?

1. What are the screens?




Description

This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (gazissax@netcom.com) with numerous contributions by others.

1. What are the screens?

1. This FAQ covers questions about two similar tests that are available for
pregnant women. The first, AFP, is a measurement of alpha-fetoprotein. The
second test is newer and combines the AFP test with measurements of two
hormones, unconjugated estriol and human chorionic gonadotropin. It is
often called the "triple screen".
-----------------------------

Addition by Dr. Tim Reynolds:

The triple test is a mis-nomer. The test combines AFP with a variety of
different assays and no definite proof of which is best currently exists In
fact they are all about the same so far as data available allows us to
tell. Combinations available are:

AFP + total HCG

AFP + Free beta HCG (free beta is a subsection of the HCG molecule)

AFP + total HCG + unconjugated estriol (uE3)

AFP + free beta HCG + uE3

AFP + free beta HCG + uE3 + Free alpha HCG

AFP + free beta HCG + uE3 + neutrophil alkaline phosphatase

The detection rates reported for these different combinations all suggest
Detection rate is about 60% and differences of 1-2% exist between different
combinations: However the data available is not sufficient to determine
which combination is better. Other evidence suggests that as the number of
analytes used to estimate risk increases, the errors become multiplied and
the result becomes less accurate. It is probably better therefore to use
the simpler screening combinations (AFP + total or free beta HCG) and not
bother with uE3, free alpha or any of the other markers. If you are told
that you can have a 2-marker test or a 3-marker test if you pay more: opt
for the 2-marker test.
-----------------------------
Both tests use a blood sample from the mother at a specific time during
pregnancy. The AFP can be done during weeks 16 to 18 LMP, the triple screen
can be done from weeks 15 to 18 LMP.
-----------------------------

Addition by Dr. Tim Reynolds:

The test can be done using AFP + free beta from 11 weeks, but the
diagnostic test has a very high rate of miscarriage (up to 5%) so it is
best to wait until 14 weeks (at least). The accuracy of the test is
affected by gestation dating: It is best to have an ultrasound scan before
the screen to confirm your gestation dates.
-----------------------------
An elevated level of AFP in the sample indicates an increased risk of
neural tube defects in the fetus. A depressed level of AFP indicates an
increased risk of Down's syndrome. The triple screen uses levels of all
three substances to indicate increased risk of Down's. This is considered a
more accurate screen for Down's.
-----------------------------

Addition by Dr. Tim Reynolds:

Down's syndrome is associated with decreased AFP and uE3, and increased
HCG. A combination of low AFP and high HCG is particularly significant. The
maths of calculating results is quite complex but also takes into account
the maternal age.
-----------------------------
Neither test can tell you if something is wrong with your baby. Only a
diagnostic test (such as chorion villi sampling or amniocentesis) can tell
definitively if something is wrong. The AFP and the triple screen help to
identify pregnancies that are at increased risk.

BEING AT INCREASED RISK DOES NOT AUTOMATICALLY MEAN YOUR BABY HAS A DEFECT!
Most women who find themselves in the elevated risk category give birth to
healthy, normal children. This fact is often missed when interpreting the
results of this screen.
-----------------------------

Addition by Dr. Tim Reynolds:

In young women, the predictive value of a +ve Downs screen result is about
1%: i.e. only 1 in every 100 young women with a +ve result will be carrying
a Down's fetus. In older women, the +ve pred. value is about 4%.
-----------------------------

 

Continue to:













TOP
previous page: 2d. My HCG level is not rising as quickly as it is supposed to? What is the normal range, and how worried should I be?
  
page up: Pregnancy Screening FAQ
  
next page: 2.a What is apha-fetoprotein?